1. Field of the Invention
The present invention relates to an illumination optical system used in a liquid crystal video projector in which three primary color light components are combined together by a cross dichroic prism so as to project a color image onto a screen, and the liquid crystal video projector using the same. In particular, the present invention relates to an illumination optical system having two lens arrays each comprising a plurality of linear convex lens arrays arranged as being shifted column by column from each other in a predetermined direction, and a polarization-converting optical system comprising a polarizing beam splitter array and a half-wavelength phase having a plurality of half-wavelength phase films arranged like stripes; and a liquid crystal projector apparatus equipped with this illumination optical system.
2. Description of the Prior Art
Known in liquid crystal projector apparatus is an illumination optical system in which an integrator optical system comprising two lens arrays and a polarization-converting optical system are combined together in order to efficiently utilize light from a light source. Such an illumination optical system is disclosed, for example, in Japanese Unexamined Patent Publication No. 8-304739.
The above-mentioned illumination optical system comprises a first lens sheet, constituted by a plurality of condenser lenses each having a rectangular outer shape, for collecting light emitted from the light source, in order to form a plurality of light source images; and a second lens sheet, disposed near the position where the plurality of light source images are formed, comprising a condenser lens array, a polarization-separating prism array, a half-wavelength phase plate, and an exit-side lens.
In this illumination optical system, randomly polarized light radiated from the light source is converged by the first lens sheet onto a predetermined area of the polarization-separating lens array, and then is spatially separated into two kinds of polarized light (P and S polarized light components). Thereafter, each polarized light component is guided to a predetermined area of the half-wavelength phase sheet, so that one kind of polarized light is converted into the other kind of polarized light. As a consequence, the randomly polarized light radiated from the light source can irradiate an illumination area while having been turned into substantially one kind of polarized light. When such an illumination optical system is used in a liquid crystal projector, almost all of the light can be utilized, whereby efficiency in light utilization would improve, thus allowing brighter projection images to be obtained.
Further, in the second lens sheet, the two kinds of polarized light emitted from the polarization-separating prism array are separated from each other in conformity to the laterally long rectangular form of the illumination area. Consequently, light quantity would not be wasted, thus allowing illumination efficiency to improve.
On the other hand, an illumination optical system using a cross dichroic prism as the color-combining means of the projector apparatus and comprising two multi-lens arrays is disclosed, for example, in Japanese Unexamined Patent Publication No. 9-90510.
The cross dichroic prism is formed by four pieces of prisms cemented together by way of thin films having predetermined reflection characteristics, and can be employed as a simple, small-sized color-combining means in a projector apparatus. Due to its precision in manufacture, however, a plurality of shadows may occur in the projected image.
The illumination optical system disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 9-90510 yields a favorable image by preventing these shadows from becoming evident.
Such shadows may occur because: 1) due to unevenness in thin films near the vertical angle ridge perpendicularly located at the center of the cross dichroic prism, the reflection characteristic of the reflecting film may vary; 2) the form-processing precision of each prism is so low that a slight gap may be formed between the cemented surfaces; 3) a level difference may occur between reflecting surfaces to be flatly cemented together at the time of bonding; and so on. When luminous fluxes, from a plurality of directions, pass through the cross dichroic prism having such a problem in precision, near the cementing ridge in particular, the light is eclipsed due to such defects, thus yielding shadows.
In the illumination optical system disclosed in Japanese Unexamined Patent Publication No. 9-90510, the respective convex lenses of the multi-lens arrays are staggered, so that such a plurality of shadows formed in the projected image become unremarkable on the image. Even when there is a defect at the center part of the cross dichroic prism, the density of the shadows appearing on the image is lowered, whereby the image quality is improved and the defect in the cross dichroic prism can be compensated for.
As mentioned above, the illumination optical system such as that disclosed in Japanese Unexamined Patent Publication No. 8-304739 in which an integrator optical system and a polarization-converting optical system are combined together, efficiency in light utilization can be improved, so as to yield a bright projection image. When a cross dichroic prism is used as the color-combining means in a projector apparatus, the problem of shadows mentioned above would similarly occur in the projector apparatus using this illumination optical system as well.
When the respective convex lenses of the lens arrays are staggered as in the case of the illumination optical system disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 9-90510, the shadows can be made thinner, so as to improve the image quality. Nevertheless, since this illumination optical system is not combined with a polarization-converting optical system, bright images may not efficiently be obtained in the liquid crystal projector apparatus.